High performance solid propellant fueled rocket motors require burning rate catalysts to achieve fast burn rates. Presently, n-hexylcarborane (NHC) is considered to be one of the most suitable burning rate catalysts for solid propellant fuels. NHC production involves reacting 1-octyne with decaborane-14. The price and quantity limiting factor in the supply of NHC is the lack of an industrial process for synthesizing large quantities of decaborane inexpensively.
Thus, the key element in upgrading a weapon system to meet the desired level of performance for its propulsion subsystem is the use of the carborane burning rate modifier, NHC in the propellant. NHC, while extremely effective, is very costly, currently around $2,000/lb. The high cost is due chiefly to the cost in preparing a precursor, decaborane (B.sub.10 H.sub.14), from diborane (B.sub.2 H.sub.6).
The present production of decaborane-14 (B.sub.10 H.sub.14) from B.sub.2 H.sub.6 by pyrolysis methods lead to the excitation of all degrees of freedom of the reactant molecule. Both external (translational) and internal (electronic, vibrational, and rotational) degrees of freedom are usually in thermodynamic equilibrium. In addition to there being an unproductive waste of energy, reactions with equilibrium excited molecules characteristically proceed in the direction of breaking of the weakest bond, completing a considerable percent of back reactions, completing many side reactions, and producing polymers, many of which are not the desired product. The percent yield of the desired product is low since a number of boron polymers or undesirable products are formed. Also, separation of the desired product leads to complexities in the chemical engineering process for the usual commercial source method for B.sub.10 H.sub.14.
A process which does not involve pyrolysis or producing decaborane-14 (B.sub.10 H.sub.14) is disclosed in U.S. Pat. 3,489,517 by Shore et al. The general procedure for this process comprises reacting together any of the alkali metal (Li, Na, K)B.sub.5 H.sub.8 salts (i.e., an alkali metal pentaborane-8) with diborane in an alkyl ether (R.sub.2 O) solvent including diethyl ether, glyme (1,2-dimethoxyethane) and diglyme[bis(2-methoxyethyl)ether]. Completion of the reaction is indicated when hydrogen evolution ceases. The desired decaborane-14 is recovered from the reaction mixture.
More recently, U.S. Pat. Nos. 4,115,520 and 4,115,521 issued to Dunks et al. on Sept. 19, 1978, disclose the process for preparation of compounds MB.sub.11 H.sub.14 (wherein M is a monovalent cation) and the process for preparation of B.sub.10 H.sub.14 by oxidation of the B.sub.11 H.sub.14.sup.(-) ion of the compound MB.sub.11 H.sub.14. These patents together provide disclosures for the preparation of B.sub.10 H.sub.14 by nonpyrolytic methods.
The improvements provided by the nonpyrolytic processes by Shore et al and Dunks et al are recognized when compared with the pyrolytic processes of the earlier prior art. The yield of 25% based on the B.sub.5 H.sub.9 used in the Shore et al process indicates that an improved method which is more direct and which provides a higher percent yield would be an advantageous advancement in the state-of-the art. Also, the Dunks et al process which employs a B.sub.11 H.sub.14.sup.- ion must be carried out in a liquid medium which is unreactive toward the oxidants and the boron-containing reagents and products, and which facilitates contact of the B.sub.11 H.sub.14.sup.- ion with the oxidatively active moiety of the oxidation agent. Again, one recognizes that this process which requires removal of one boron atom from B.sub.11 H.sub.14.sup.- ion to produce B.sub.10 H.sub.14 by oxidation in an aqueous medium could be improved by a method where the final reaction is conducted between all solid reactants to produce B.sub.10 H.sub.14 in high yield by adding a single boron atom to an anionic starting material by a hydride ion abstraction procedure. The advantages of a method for the conversion of pentaborane-9 to decaborane-14 and the separation of decaborane-14 by a sublimation process from the solid reaction medium in which it is formed are readily recognized over a solvent process, particularly where water and non-polar organic media are required in the reaction and separation techniques.
A nonpyrolytic method for producing B.sub.10 H.sub.14 from a starting material containing a greater number of boron atoms would be attractive, particularly, if the method is a simple method which results in good yields of B.sub.10 H.sub.14.
A method for producing B.sub.10 H.sub.14 from a starting material which is already available in quantity reserves would be particularly attractive in view of present day financial costs to produce B.sub.10 H.sub.14 by pyrolysis methods.
An object of this invention is to provide a systematic approach to boron hydride syntheses wherein the syntheses relate to hydride ion abstraction from certain boron hydride anions to yield as one of the final products a neutral boron hydride which contains one more boron atom than the anionic starting material.
Another object of this invention is to provide a method for the conversion of pentaborane-9 to decaborane-14 and its separation by a sublimation process from the solid reaction medium.